In WDM, optical carriers having different wavelengths are individually modulated. The transmission capacity of an optical-fiber transmission line is increased according to the number of wavelengths, which define WDM channels. A large number of WDM channels can be supported if the channels are closely spaced (e.g. 50 GHz).
WDM channels are transmitted over a single waveguide and demultiplexed by diffraction or filtering such that each channel wavelength is routed to a designated receiver. Optical amplifiers (such as doped fiber amplifiers) simultaneously amplify the optical channels, facilitating use of the WDM protocol.
Dense WDM systems require special add/drop multiplexer filters (ADM filters) to add and drop channels (wavelengths). At each node in the system, certain wavelengths on one fiber may be dropped onto a second fiber, and channels from a third fiber may be added. The number of WDM channels determines the number of nodes. However, dispersion, four-wave mixing, wavelength drift of transmission sources, and the difficulty of separating closely spaced wavelengths at a receiver restrict the number and spacing of the channels.
I. Dispersion
The principal limiting factor in high-rate communication systems is chromatic dispersion. Chromatic dispersion is characterized by a widening in the duration of pulses as they travel through a fiber. Dispersion is caused by the dependence of the effective index of the fiber on the wavelength of each wave transported. The variation in the index of refraction with respect to wavelength causes different channel wavelengths to travel at different speeds. This phenomenon is also known as group-velocity dispersion (GVD).
Increasing data-transmission rates severely limits the transmission distance because of the waveform distortion caused by GVD in optical fibers. Furthermore, when the transmission speed is increased, the optical power for transmission needs to be increased to maintain the required received optical-power levels.
Many techniques and devices have been devised to counter the effects of GVD. The goal of dispersion compensation is to change a nonlinear channel into a linear channel (at least for a specific range of wavelengths) in order to achieve the capacity of a linear channel. None of these references make use of the nonlinearity of the channel to achieve capacity that may exceed the theoretical limitation of a linear channel.
U.S. Pat. No. 4,677,618 describes a method of compensating for distortion of WDM data by a dispersive medium. Dispersion-compensation techniques include providing lengths of dispersion-compensating line in an optical network (U.S. Pat. No. 5,361,234) and providing dispersion-slope compensation, such as disclosed by J. A. R. Williams et al. in IEEE Photonics Technology Letters, Vol. 8. p. 1187 (1996) and K. Takiguchi et al. in Electronics Letters, Vol. 32 p. 755 (1996). However, these dispersion-compensation methods have relatively limited effectiveness with respect to bandwidth.
To minimize the dispersion value of optical signals in fiber, work is currently under way to transmit signals in the 1.55-μ range in a dispersion-shifted fiber. U.S. Pat. Nos. 5,943,151, 5,898,714, 5,877,879, and 5,828,478, describe methods of phase comparisons and synchronization to compensate for chromatic dispersion.
II. Other Distortions
Other types of distortion also occur. U.S. Pat. No. 5,847,862 describes shaping of amplifier outputs to offset depletion of high-frequency channels. A significant factor in signal-to-noise ratio (SNR) degradation in WDM is due to Raman crosstalk. U.S. Pat. No. 5,953,140 cancels out the effects of crosstalk by processing signals in the electrical domain after the WDM transmission has been demultiplexed. Also, smoothing the power variation of the optical signal transmitted through the fiber can reduce nonlinear effects. U.S. Pat. No. 5,589,969 addresses the problem of interference caused by four-wave mixing between different WDM signals by providing a non-periodic spacing between the signal wavelengths.
III. Wavelength Drift
WDM lasers require extremely tight manufacturing tolerances with respect to center wavelength and line width. There are significant problems with laser-wavelength drift resulting from environmental factors, such as temperature variations and aging. Wavelength drift causes substantial problems in distributed systems because each receiver needs to demultiplex signals from different transmitters and from different fiber lines, all of which independently operate under different and changing environmental conditions. Conventional WDM systems require strict manufacturing and environmental controls to stay within tolerance.
Efforts to improve WDM systems have focused on improving the wavelength stability of the transmitter lasers. U.S. Pat. No. 5,943,152 describes a method for stabilizing the wavelength of an optical source. U.S. Pat. No. 5,838,470 addresses the problem that WDM transmitters and receivers must be precisely tuned to predetermined fixed wavelengths. In the '470 patent, each transmitter transmits a synchronization signal that the receiver uses to determine the wavelength of the signals. These signal wavelengths are stored in a lookup table.
U.S. Pat. No. 5,894,362 includes a decoupling unit for decoupling a portion of the WDM signal from a fiber as a monitoring signal. A monitoring unit determines the spectrum of the WDM signal with an optical spectrum analyzer. The monitoring unit uses the spectrum information to control light sources such that the wavelength is constant for each signal. The monitoring unit also detects SNR and signal power, maintains received power levels, counts the number of channels in a WDM signal, measures the spacing between wavelengths, monitors any changes in the spectrum, and controls optical amplifiers to achieve a desired noise figure or maintain a flat gain.
U.S. Pat. No. 5,555,086 describes a sensor array used to monitor physical characteristics of an optical fiber. A two-mode signal is sent through the fiber. Each mode has a different propagation velocity to create an interference pattern at a sensor array. Changes in the interference pattern indicate changes in the physical characteristics of the fiber.
None of the references disclose a communication protocol wherein optical switching is controlled by relative frequencies and phases between multiple carrier signals. None of the references disclose an optical source that generates multi-frequency carrier signals that are identically affected by frequency drifts. None of the references describe a method of generating information signals from relative frequency and phase relationships between carrier signals in order to reduce the effects of carrier-frequency drifts.
IV. Insertion Loss
Insertion loss limits the number of sources that can be coupled into a fiber. Insertion loss limits the number of wavelength channels produced by multiple optical sources. U.S. Pat. No. 5,589,969 describes an array of passive resonant cavities (Fabry-Perot filters) that reduces insertion loss. The cavities can be used to demultiplex several received signal wavelengths and multiplex several different wavelengths into a single multichannel laser signal.
V. Switching
Virtual point-to-point connections are achieved with WDM when different frequency channels are routed to different locations. However, the number of locations is limited to the number of WDM channels.
Wavelength demultiplexers with the smallest channel spacing are the most difficult to fabricate. Small channel spacing also results in cross talk between channels. U.S. Pat. No. 5,680,490 uses a multi-stage array of demultiplexers and bandpass filters. The array still requires the use of expensive demultiplexers to separate closely spaced channels, and it increases the number of demultiplexers.
U.S. Pat. No. 5,778,118 describes the use of optical add-drop multiplexers for WDM systems. U.S. Pat. No. 5,854,699 discloses a switching technique involving the modulation of data and control signals onto the same optical carrier. U.S. Pat. No. 5,940,208 discloses a switchable fiber-optic device.
None of the references describe an interference relation between different carriers that can be used to receive baseband information signals.
VI. Frequency-Shifted Feedback
In the Optics Letters article “Broadband Continuous Wave Laser,” applicant described a laser design that utilizes a traveling-wave frequency-shifted feedback cavity (FSFC) to circulate light through a gain medium. Light circulating through the FSFC is frequency shifted by an acousto-optic modulator (AOM) upon each pass through the cavity. A unique characteristic of this cavity is that, unlike a Fabry-Perot cavity, it does not selectively attenuate signal frequencies. In the thesis “A New Method for Generating Short Optical Pulses,” applicant describes how an optical signal propagating through an FSFC is spread in frequency to generate broadband lasing. The amount of frequency spreading is proportional to the number of times that light circulates through the cavity. In the Applied Physics Letters article “Optical Pulse Generation with a Frequency Shifted Feedback Laser,” applicant describes an interference condition in which the broadband output of a laser produces short optical pulses, which have a frequency that is related to the RF-shift frequency of an AOM.
VII. Carrier Interferometry
In U.S. Pat. No. 5,955,992, U.S. patent application Ser. No. 09/393,431, and PCT Pat. Appl. No. WO99/41871, which are hereby incorporated by reference, applicant describes a multicarrier protocol that uses interference characteristics between the carriers to convey baseband information signals. This protocol is known as Carrier Interference Multiple Access (CIMA). The CIMA protocol involves different carriers that are redundantly modulated and selected to provide a predetermined phase relationship. However, this redundancy does not diminish the bandwidth efficiency. In fact, superior bandwidth efficiency is achieved because the CIMA carriers combine in the time domain to produce short impulses. This results in orthogonality in the time domain.
In wireless systems, CIMA signals can be processed as both low-bandwidth and high-bandwidth signals simultaneously. This mitigates the effects of both intersymbol interference and multipath fading. The frequency-diversity of CIMA signals greatly reduces transmission power requirements and eliminates the effects of jamming interference. CIMA also simplifies multi-user detection. Unlike other spread-spectrum techniques that distribute user interference to all users in a communication system, CIMA confines user interference to adjacent users in the time-domain, thus making it simple to cancel. CIMA can also be used to construct other protocols, such as Direct-Sequence CDMA (DS-CDMA). However, unlike DS-CDMA, which requires fast serial processing to create a direct-sequence spreading code, CIMA uses a slow type of parallel processing to generate the exact same codes (but with all of the additional benefits of a CIMA signal). Slow parallel processing makes CIMA systems simpler and less costly to implement.
Another benefit of CIMA is the ability to transmit and receive signals in non zero-phase space, which is an interference condition at one or more time instants within a symbol interval in which the carrier signals corresponding to one data symbol cancel where similar carrier signals corresponding to a different data symbol constructively combine. CIMA signals are detectable by receivers that are tuned to the carrier frequencies and carrier-phase relationships of the carrier signals.
Frequency diversity in the CIMA protocol also enables spatial demultiplexing of the received CIMA signals without antenna arrays. Different carriers have different spatial gain distributions (in a multipath fading environment) due to their differences in frequency. Therefore, each transmitted signal has a unique spatial gain distribution represented by the individual complex amplitudes of its component carriers. A training sequence can be used to determine (and adjust) the spatial gain distributions of the CIMA carriers at one or more receivers. Received CIMA signals are then separated into their carrier components. The complex amplitude of each component includes spatial-gain terms and unknown information signals. The amplitudes are extracted from each carrier and processed in a cancellation system (described in U.S. patent application Ser. No. 08/279,050, PCT Appl. No. WO95/03686, U.S. patent application Ser. Nos. 08/862,859, 09/324,206, and 09/347,182, which are all incorporated by reference), which solves for the unknown signals.
VIII. Redundancy
The transmission protocol used in the present invention is related to CIMA. Although redundancy in transmission dimensions (such as frequency) is well known in the prior art, CIMA is unique because it provides benefits of frequency diversity via redundant transmissions in the frequency domain without sacrificing bandwidth efficiency.
U.S. Pat. No. 5,940,196 describes an optical-communication system that transmits the same information on two optical carrier signals having different frequencies and combines the signals at a receiver to increase the carrier-to-noise ratio. However this benefit is achieved at the expense of reducing bandwidth efficiency.
IX. Time-Division Multiplexing
The method of generating time-domain CIMA signals is distinct from Optical Time Division Multiplexing (OTDM). U.S. Pat. Nos. 5,654,812 and 5,331,451 describe OTDM, which involves using time-domain processes to generate information pulses modulated onto a carrier signal. Each carrier signal is transmitted and optically demultiplexed at a receiver.
X. Pulse Formation
An ideal optical pulse consists of an envelope of a fixed-frequency carrier that is modulated according to a given temporal profile. G. P. Agrawal (Nonlinear Fiber Optics, Academic Press Inc. section 3, paragraph 2, pages 54–64) shows that in the absence of chirping, the spectral amplitude is minimized with respect to the pulse duration. Chirping is a frequency variation of the optical carrier enveloped by the laser-generated pulse.
Soliton transmission techniques are used to mitigate dispersion. Soliton-type pulses cause nonlinear variations in the refraction index of an optical medium due to the high power of the solitons, resulting in a counteracting of the effects of chromatic dispersion. The objective of time-domain dispersion-compensation techniques is to reduce the amount of pulse spreading in an optical fiber.
None of the prior-art references teach a method for using dispersion to reduce the duration of a pulse as it propagates through an optical medium.